Memory systems and methods including training, data organizing, and/or shadowing

ABSTRACT

Described embodiments include memory systems that may shadow certain data stored in a first memory device (e.g. NAND flash device) onto a second memory device (e.g. DRAM device). Memory systems may train and/or re-organize stored data to facilitate the selection of data to be shadowed. Initial responses to memory commands may be serviced from the first memory device, which may have a lower latency than the second memory device. The remaining data may be serviced from the second memory device. A controller may begin to access the remaining data while the initial response is being provided from the first memory device, which may reduce the apparent latency associated with the second memory device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry of, and claims priority toand the benefit of, International Application No. PCT/CN2013/000285,filed on Mar. 14, 2013 and entitled “MEMORY SYSTEMS AND METHODSINCLUDING TRAINING, DATA ORGANIZING, AND/OR SHADOWING.”

TECHNICAL FIELD

Embodiments of the invention relate generally to semiconductor memory,and examples of memory systems which may shadow some data fromNAND-based flash memory in dynamic random-access memory (DRAM) aredescribed.

BACKGROUND

Electrically erasable and programmable memory devices having flashmemory cells are found in a wide variety of electrical devices. Anexample flash memory cell, also known as a floating gate transistormemory cell, may be similar to a field effect transistor, having asource region and a drain region that is spaced apart from the sourceregion to form an intermediate channel region. A floating gate, whichmay be made of doped polysilicon, may be disposed over the channelregion and may be electrically isolated from the channel region by alayer of gate oxide. A control gate may be fabricated over the floatinggate, and it may also be made of doped polysilicon. The control gate maybe electrically separated from the floating gate by a dielectric layer.Thus, the floating gate is “floating” in the sense that it may beinsulated from the channel, the control gate and all other components ofthe flash memory cell.

An example flash memory cell may be programmed by storing charge on thefloating gate. The charge thereafter may remain on the gate for anindefinite period even after power has been removed from the flashmemory cell. Flash memory cells may therefore be referred to asnon-volatile. Charge may be stored on the floating gate by applyingappropriate voltages to the control gate and the drain or source. Forexample, negative charge can be placed on the floating gate by groundingthe source while applying a sufficiently large positive voltage to thecontrol gate to attract electrons, which tunnel through the gate oxideto the floating gate from the channel region. The voltage applied to thecontrol gate, called a programming voltage, and the duration that theprogramming voltage is applied as well as the charge originally residingon the floating gate, determine the amount of charge residing on thefloating gate after programming.

An example flash memory cell may be read by applying a positive controlgate to source voltage having a magnitude greater than a thresholdvoltage. The amount of charge stored on the flash memory cell maydetermine the magnitude of the threshold voltage that must be applied tothe control gate to allow the flash memory cell to conduct currentbetween the source and the drain. As negative charge is added to thefloating gate, the threshold voltage of the flash memory cell increases.During a read operation, a read voltage may be applied to the controlgate that is large enough to render the cell conductive if insufficientcharge is stored on the floating gate, but not large enough to renderthe cell conductive if sufficient charge is stored on the floating gate.During the read operation, the drain, which is used as the outputterminal of the cell, may be precharged to a positive voltage, and thesource may be coupled to ground. Therefore, if the floating gate of theflash memory cell is sufficiently charged, the drain will remain at thepositive voltage. If the floating gate of the flash memory cell is notsufficiently charged, the cell will ground the drain.

Before a flash memory cell can be programmed, it must be erased in somecases by removing charge from the floating gate. The cell can be erasedby applying a gate-to-source voltage to the cell that has a polarityopposite that used for programming. Specifically, the control gate maybe grounded, and a large positive voltage applied to the source to causethe electrons to tunnel through the gate oxide and deplete charge fromthe floating gate. In another approach, a relatively large negativevoltage is applied to the control gate, and a positive voltage, such asa supply voltage, is applied to the source region.

A typical flash memory device includes a number of flash memory cells,which may be arranged in rows and columns. Two common types of flashmemory array architectures are the “NAND” and “NOR” architectures, socalled for the logical form in which the basic flash memory cellconfiguration of each is arranged. NOR flash may generally act as a NORgate—e.g. when a word line is brought high, a corresponding transistormay act to pull an output bit line low. NAND flash may generally includefloating-gate transistors connected in a way resembling a NAND gate—e.g.several transistors may be connected in series, and only when all wordlines are high may a bit line be pulled low.

Generally, NOR flash memory may provide a faster read response than NANDflash memory. Moreover, NAND flash memory may require housekeepingprocesses to refresh the memory and repair bad blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system according to anembodiment of the present invention.

FIG. 2 is a flowchart illustrating an example method of servicing amemory command in accordance with an embodiment of the presentinvention.

FIG. 3 is a flowchart of an example method for shadowing data from NANDflash devices to DRAM devices in accordance with an embodiment of thepresent invention.

FIG. 4 is a representation of at least a portion of a logical addressspace in accordance with an embodiment of the present invention.

FIG. 5 is a representation of at least a portion of a physical addressspace on a NAND flash device in accordance with an embodiment of thepresent invention.

FIG. 6 is a schematic illustration of a mapping table in accordance withan embodiment of the present invention.

FIG. 7 is a schematic illustration of NAND flash and DRAM address spacesarranged in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without various of these particular details. In someinstances, well-known circuits, control signals, timing protocols, andsoftware operations have not been shown in detail in order to avoidunnecessarily obscuring the described embodiments of the invention.

Many present systems currently utilize NOR flash and masked read onlymemory (ROM) in applications requiring a small latency, Suchapplications include, but are not limited to, operation of gamblingmachines that may require highly random, sudden accesses to stored datacorresponding to video and/or images. NAND flash may be desirable due toits generally lower cost, however the relatively higher latencyassociated with NAND flash may be prohibitive in some examples.Moreover, NAND flash may require housekeeping operations be performed atunpredictable times, which may result in a wait of up to seconds fordata retrieval which, again, in some applications may be undesirable.Embodiments of the present invention accordingly provide memoryarchitectures utilizing both NAND flash and DRAM memory in a mannerwhich may improve latency performance for some applications. Forexample, certain data stored in the DRAM memory may be stored in theNAND flash memory, in some examples, housekeeping operations may beperformed only following a power-up operation of the memory system.

While NAND flash and DRAM memory are used to describe example systemsherein, in other examples other types of memory may be used. Generally,embodiments of the present invention may be used to improve theeffective latency associated with one type of memory. Accordingly,portions of data from one set of memory devices (e.g. NAND flash) may beshadowed into another set of memory devices (e.g. DRAM) generally havinga lower latency than the first set of memory devices. The shadowedportion of data may be provided to the host while remaining data isretrieved from the higher latency memory devices (e.g. NAND flash).

Examples of particular applications, e.g. gambling, are described hereinby way of example. It is to be understood that embodiments describedherein may be used in any of a variety of applications including, butnot limited to, cameras, phones, wireless devices, displays, chip sets,set top boxes, gaming systems, vehicles, and appliances.

FIG. 1 is a schematic illustration of a system according to anembodiment of the present invention, The system 100 may include aprocessor-based system 105 that may be in communication with a memorysystem 107 over an interface 109, The interface 109 may be implementedusing, for example, a serial advanced technology attachment (SATA)interface or other memory system interface. The processor-based system105, as described above, may include one or more electronic devices,including but not limited to, computers including desktop, laptop, andtablet computers, gambling machines, cameras, phones, wireless devices,displays, chip sets, set top boxes, gaming systems, vehicles, andappliances.

The memory system 107 may include a controller 111. The controller 111may be coupled to an interface 113 for communication with one or moreNAND flash devices 115. The interface 113 may, for example, beimplemented using a bus or other electronic communication conduit anddata may be communicated over the interface 113 in a manner compatiblewith the NAND flash devices 115, such as, but not limited to, a doubledata rate (DDR) protocol. The controller 111 may further be coupled toan interface 121 for communication with one or more DRAM devices 125.

Generally, any number of NAND flash devices 115 may be used, includingone or more NAND flash devices. Eight NAND flash devices 115 a-h areshown in FIG. 1. Moreover, any number of DRAM devices 125 may be used,including one more DRAM device. Four DRAM devices 125 a-d are shown inFIG. 1 by way of example. The NAND flash devices 115 may generally beused to store data for use by the processor-based system 105. The datamay include, for example, image or video data. A portion of the datastored on the NAND flash devices 115 may be shadowed onto the DRAMdevices 125. In some examples, portions of the data stored on the NANDflash devices 115 is also stored on the DRAM devices 125. A mappingtable 131 may be stored in one or more of the DRAM devices 125 toindicate which data is shadowed in the DRAM devices 215 from the NANDflash devices 115. In this manner, when the processor-based system 105seeks to access certain data, in some examples, the data may be accessedfrom the DRAM devices 125, and remaining data may be accessed from theNAND flash devices 115 at least in part during a time the data is beingaccessed from the DRAM devices 125. Accordingly, by the time an initialportion of the requested data is read from the DRAM devices 125, thelatency in accessing the remaining data on the NAND flash devices 115may be over, and the remaining data read from the NAND flash devices 115may be ready for use by the processor-based system 105. In this manner,data shadowing on the DRAM devices 125 may mask read latency associatedwith the NAND flash device 115.

Communication over the interfaces 113 and 121 in FIG. 1 may generally bearranged in any manner, including any number of channels. In the exampleof FIG. 1, the interface 113 may be implemented using a four channelNAND interface. The eight NAND devices 115 a-h may be arranged such thattwo NAND devices (e.g. dies) are accessible over each channel. In otherexamples, other numbers of channels and/or devices per channel may beused. In the example of FIG. 1, the interface 121 may also beimplemented using two devices (e.g. dies) per channel, and the interface121 may support two channels.

FIG. 2 is a flowchart illustrating an example method of servicing amemory command (e.g., read command) in accordance with an embodiment ofthe present invention. In block 202, a memory command may be received.Referring back to FIG. 1, the memory command may be received by thecontroller 111 from the processor-based system 105. Responsive to thememory command, referring again to FIG. 2, an initial data chunk may beread from a DRAM device in block 204. For example, the controller 111 ofFIG. 1 may read an initial data chunk corresponding to the datarequested in the memory command from one or more of the DRAM devices125. The size of the initial data chunk stored in the DRAM may vary indifferent embodiments, however, generally the size is selected to besufficiently large that by the time the initial data chunk is providedto the processor-based system, a next data chunk may arrive from theNAND devices with an acceptable latency, e.g. the next data chunk may beready for provision to the processor-based system once the initial datachunk has been provided to that processor-based system (e.g. host).

The initial data chunk may be provided to the processor-based system(e.g. host) in block 206. During at least a portion of the time that theinitial data chunk is being provided to the processor-based systemrequesting the data, a next data chunk may be being accessed in one ormore NAND devices in block 208. The next data chunk may not becompletely accessed during the time blocks 204 and/or 206 are beingperformed, but the next data chunk may at least be being accessed duringthat time. Accessing a next data chunk from one or more NAND devices mayaccordingly occur during at least a portion of a time that the initialdata chunk is being provided to the host in block 206 and/or during atleast a portion of time that the initial data chunk is being read fromone or more DRAM devices in block 204. In this manner, the latencyassociated with accessing one or more NAND devices may be at leastpartially masked by the at least partially concurrent reading and/orproviding of the initial data chunk to the requesting processor-basedsystem. The next data chunk may be provided to the host in block 210.Although not shown in FIG. 2, the blocks 208 and 210 may be repeateduntil the memory command has been serviced. For example, a further datachunk may be read from one or more NAND devices during a portion of timethat the next data chunk was being accessed in one or more NAND devicesin block 208 and/or during a portion of time that the next data chunkwas provided to the host in block 210. By the time the next data chunkhas been provided to the host at the end of the block 210, then, thefurther data chunk may be ready for provision to the host.

Referring to FIG. 1, the system 100 may implement the process shown inFIG. 2, for example the controller 111 may read an initial data chunkfrom one or more of the DRAM devices 125 and provide the initial datachunk to the system 105. During at least a portion of time that thecontroller 111 is reading the initial data chunk and/or providing theinitial data chunk to the system 105, the controller 111 may read a nextdata chunk from one or more of the NAND devices 115. The controller 111may time these activities such that when the processors-based system 105has received the first data chunk, the next data chunk is ready forprovision to the processor-based system 105 with an acceptable amount,e.g. no additional, latency.

In one example of an implementation of the system of FIG. 1, aprocessor-based system 105 may require a throughput of the memory system107 be greater than or equal to a specified throughput, 400 MB/s in oneexample. As described above, data may be read from the DRAM devices 125and/or the NAND flash devices 115 in chunks. The chunks may have aparticular size, e.g. 128 kB in one example. Using these exemplarynumbers, the memory system 107 should be able to provide a 128 kB chunkto the processor-based system 105 within 320 μs. If the first chunk(e.g. 128 kB) is located in the DRAM, the time, Tr, for the controller111 to analyze the memory command, address, read the data, and begin theresponse to the processor-based system 105 may be minimal. To minimizethe apparent latency from accessing the requested data, the controller111 should be able to read the next 128 kB chuck from NAND flash within320 μs -Tr. Tr may, in one example, be less than 10 μs. Accordingly, thecontroller 111 may have more than 310 μs to prepare the next 128 kB datachunk for provision to the processor-based system if it is accessing theNAND flash data chunk at approximately the same time as it is accessingthe first data chunk from the DRAM. This timing may be met, for example,by a 4 channel solid state drive (SSD) controller and 16 kB/pagesingle-level cell (SLC) NAND memory product in one example, which may beable to achieve a read of 32 kB/channel and complete error correctingcode (ECC) correction on those channels within the requisite 310 μs.

Embodiments of the present invention accordingly may advantageouslyshadow a first portion (e.g. chunk) of data to be accessed in DRAMdevices while the remainder of the data to be accessed may be stored inNAND flash devices. Advantageously, embodiments of memory systemsdescribed herein may store the first data chunks of data that isgenerally accessed as a group in one or more of the DRAM devices. Forexample, the first data chunk of a file (e.g. a video, image, document,program, or the like) may be stored in the DRAM while the remaining dataof the file may be stored in the NAND flash. In this manner, when theprocessor-based system 105 of FIG. 1 provides a memory command to thememory system 107, the first data chunk corresponding to the memorycommand may be found in the DRAM while the remaining data may be foundin the NAND flash.

While examples described herein include examples where a first portionof data to be accessed (e.g. one chunk) of data may be shadowed in DRAMdevices, in other examples other portions of data may be shadowed (e.g.multiple initial data chunks). Generally, the more data that isshadowed, the longer a latency of the NAND flash devices that may beeffectively hidden, however, the greater amount of storage that may berequired in the DRAM for shadowed data.

Examples of how the shadowing may be implemented are further describedherein. FIG. 3 is a flowchart of an example method for shadowing datafrom NAND flash devices to DRAM devices in accordance with an embodimentof the present invention. Generally, it may be desirable to knowstarting addresses and lengths of data to be accessed in the NAND flashdevice. In read only systems (e.g. gambling machines), the data may bepre-programmed into the memory system and may not change duringoperation. Accordingly, in some embodiments, the location and length ofdata to be accessed (e.g. the starting address and length of data ofindividual files), may be known ahead of time, and a data structure ofstarting addresses and lengths may be provided to or stored in thememory system. For example, such a data structure may be stored in oneor more of the NAND flash devices 115 of FIG. 1 or the DRAM devices 125of FIG. 1. The data structure of starting addresses and lengths may beimplemented as a list, for example. Although other data structures maybe used as well, the data structure of starting addresses and lengthswill be referenced as a “list.”

The list of known starting addresses and lengths may be accessed inblock 304 of FIG. 3 and may be used to identify chunks of datacorresponding to the known starting addresses to be shadowed to the DRAMdevices. In some examples, a memory system, such as the system 107 ofFIG. 1, may be trained in block 302 of FIG. 3, to identify itself thelocation and length of data to be accessed in the NAND flash devices.During training in block 302, a memory controller (e.g. the controller111 of FIG. 1) may log addresses of memory commands received from a host(e.g. the processor-based system 105 of FIG. 1). By analyzing the listof addresses, the controller may itself generate the list of knownstarting addresses and lengths of data to be accessed in block 304 ofFIG. 3. In other examples, the training in block 302 may be implementedthrough other analysis of the content of the NAND flash devices in amemory system. For example, headers or other data strings indicative ofa start of a data grouping that may be requested in a memory commandfrom the processor-based system may be identified by the controllertogether with the length of the data grouping. In some examples,training may occur during a training period which may be indicated, forexample, by a signal provided from the processor-based system to thememory controller.

In block 306, data may be re-organized and a mapping table generated.The data may be re-organized for storage in the NAND flash devices in amanner that may be conducive to shadowing portions of the data in theDRAM devices. The generated mapping table may include a map of logicalto physical addresses. The logical addresses may be the addresses asspecified by, e.g. the processor-based system 105 of FIG. 1, while thephysical addresses may be the addresses for the of NAND flash devices asunderstood by, e.g. the controller 111 of FIG. 1. Two types of datare-organization may occur. In some embodiments only one type may occurand in other embodiments both may occur. In some embodiments other typesof data re-organization may also occur.

A first type of data re-organization may pack multiple anticipatedmemory accesses corresponding to data of less than a size of a datachunk together into a single data chunk. Generally, a data chunk refersto an amount of data that may be read at a same time from the NAND flashin a given memory system. With reference to FIG. 1, in an example wherethe interface 113 supports 4 channels with two die per channel and twopages per die may be accessed, and a page size of 8 bytes, the datachunk size may be 4×2×2×8=128 kB.

Generally, the data chunk may also be referred to as a super page. Thedata chunk should be sufficiently sized such that the time for the datachunk to be output from the NAND flash devices of FIG. 1 is less than orequal to the time for the same size data chunk to be output from theDRAM, as has been described above to hide or reduce the effectivelatency of the NAND devices.

Data shadowed from the NAND device to the DRAM devices may be stored indata chunks. To reduce the amount of space required for shadowing in theDRAM devices, it may be advantageous to consolidate data associated withmemory accesses less than a size of a data chunk. Accordingly, a firsttype of data re-organizing that may occur in block 306 of FIG. 3 is tomap logical addresses corresponding to data having a length less than adata chunk (e.g. 128 kB in one example) to a same data chunk. Forexample, the table generated in block 302 may include one expectedmemory access at a first logical address for data having a length of 64kB and a second expected memory access for data at a second logicaladdress having a length of 64 kB. Both the first and second logicaladdresses may be mapped to a same data chunk of the NAND flash devicessuch that the two 64 kB memory accesses are located in a 128 kB datachunk. Any combination of data accesses totaling a size of a data chunkmay be mapped to a single data chunk. For example, data associated withtwo 32 kB accesses and a 64 kB access may be re-organized to a singledata chunk. In another example, data associated with 5 16 kB accesses, a32 kB access, and two 8 kB accesses may be re-organized to a single datachunk. In this manner, the controller 111 may access the list ofstarting addresses and lengths which may be generated in block 302 tomap accesses to data having a length less than a data chunk into aconsolidated set of data chunks. As will be described further below, theconsolidated data chunks may be shadowed on one or more of the DRAMdevices, such that the data accesses for these data of a size less thanthe data chunk may be serviced from the DRAM devices.

FIG. 4 is a representation of at least a portion of a logical addressspace in accordance with an embodiment of the present invention. FIG. 5is a representation of at least a portion of a physical address space ona NAND flash device in accordance with an embodiment of the presentinvention. As shown in FIG. 4, a processor-based system (e.g. the system105 of FIG. 1) may access data by requesting data A, B, C, D, E, F, G,or H as shown. However, those data are spread out in logical addressspace and are less than a data chunk in size. During the datare-organization 306 of FIG. 3, the data may be re-organized to physicaladdress space as shown in FIG. 5 where data A, B, C, D, E, F, G, and Hare consolidated into a single data chunk.

Another type of data re-organization which may occur in block 306 is torealign data corresponding to accesses which are larger than a datachunk in size (e.g. larger than 128 kB in some examples). If the firstaddress data corresponding to an access larger than a data chunk isaligned with a data chunk, no reorganization may occur. However, if thefirst address is not aligned with a data chunk boundary, the physicaladdress of the first address may be shifted to align with a data chunkboundary, and the resulting map between the first address and thephysical address may be stored in a logical to physical address table.

The data re-organization of block 306 in FIG. 3 may be performed, forexample, by the controller 111 of FIG. 1. In some examples, thecontroller 111 may access a list of known starting addresses for memoryaccesses and lengths of data to be accessed, such as the list generatedin block 302 of FIG. 3. The controller 111 may consolidate together dataassociated with memory accesses where the data to be accessed is lessthan a size of a data chunk, and may align data associated with memoryaccesses where the data to be accessed is greater than a size of a datachunk with a data chunk. The controller 111 may generate a logical tophysical mapping table which reflects the re-organization. In someexamples, data had already been stored to one or more of the NAND flashdevices 115 and may be moved to the locations corresponding to thoseidentified in the re-organization process. In other examples, data maybe written to the NAND flash devices by the controller 111 in accordancewith the organization identified in the re-organization process of block302.

In block 308 of FIG. 3, selected data may be shadowed from one or moreof the NAND flash devices to one or more DRAM devices. For example, thecontroller 111 may shadow the selected data to the DRAM devices.Shadowing may include copying the selected data from one or more of theNAND flash devices to one or more of the DRAM devices such that the datais stored both in one or more of the NAND flash devices and in one ormore of the DRAM devices. Generally, in block 308 the selected data mayinclude 1) data chunks on one or more of the DRAM devices that areassociated with memory accesses for data having a length less than orequal to a data chunk; and/or 2) first data chunks associated withmemory accesses for data having a length larger than a data chunk. Inthis manner, memory commands for data having a length less than or equalto a data chunk may generally be serviced from one or more of the DRAMdevices. Further, memory commands for data having a length greater thana data chunk may have a first portion of the memory command servicedfrom one or more of the DRAM devices and a remainder of the memorycommand serviced from one or more of the NAND flash devices.

The shadowing in block 308 may be performed, for example, by thecontroller 111 of FIG. 1, which may access the list of known startingaddresses and lengths which may be generated in block 302 and themapping table from block 306 to select and shadow the data in block 308.The controller 111 may further update entries in the mapping table toreflect the shadowing as will be further described below.

The shadowing in block 308 may be performed in a variety of ways. In oneexample, shadowing may be initiated by receipt of a signal indicative ofshadowing which may be supplied by, e.g. a processor based system suchas the system 105 of FIG. 1. The controller may then provide a busysignal to the host indicative of shadowing beginning, and remove thebusy signal and/or provide a different signal to the host indicative ofshadowing having been completed. In some examples, the host may nottrigger shadowing again by providing the signal indicative of shadowinguntil a next time the memory system experiences a power cycle (e.g.power on).

In another example, shadowing may be performed during training, e.g. atleast in part concurrently with block 302 of FIG. 3. When a randomaccess is received by a controller during training, a relevant portionof the data may be shadowed when the data associated with the access hasnot yet been shadowed.

FIG. 6 is a schematic illustration of a mapping table in accordance withan embodiment of the present invention. The mapping table 600 may begenerated, for example, in blocks 306 and/or 308 of FIG. 3. The mappingtable 600 includes a logical to physical address map 602 and a physicalto shadow address map 604. The logical to physical address map 602 mayassociate a logical address with the physical address in NAND flashmemory where the data may be located, as may be affected by there-organization process described above. Moreover, the logical tophysical address map 602 may include a flag, referred to as an fS flagin FIG. 6. The fS flag may indicate whether or not the associatedaddress is shadowed in DRAM memory. In one example, the fS flag may beone bit in length, with 0 indicative that the logical-physical addressis not shadowed, and 1 indicative that the address is shadowed. In someexamples, the addresses used in the table 600 are block addresses.

Accordingly, referring back to FIG. 3, as data is re-organized, thecontroller 111 of FIG. 1 may record the associations between logical andphysical addresses in the mapping table 600 of FIG. 6. The mapping table600 may be stored in one or more DRAM devices 125 of FIG. 1, such as isshown by the mapping table 131, or may be stored in one or more of theNAND devices 115 of FIG. 1. In some embodiments, the mapping table 600may be stored in the DRAM devices 125 and/or the NAND devices 115.

The mapping table 600 of FIG. 6 may further include associations betweenphysical and shadow addresses 604. If data stored at a particularphysical address in the NAND flash is shadowed to DRAM, the fS flag inthe mapping table 600 may so indicate shadowing. Moreover, theassociated address of the shadowed data may be stored in the mappingtable 600.

Moreover, the physical to shadow address map 604 may include a flag,referred to as an fC flag in FIG. 6. The fC flag may indicate whether ornot the next data chunk is located in shadow memory (e.g. DRAM) or inthe NAND flash memory. In one example, the fC flag may be one bit inlength, with 0 indicative that the following data chunk is located inNAND flash, and 1 indicative that the following data chunk is located inthe shadow memory (e.g. DRAM). So, for example, if the logical addresscorresponds with a physical address where the memory to be accessed isexpected to be less than a single data chunk, the next memory commandmay also be serviced from the shadow memory (e.g. DRAM), so the fC flagmay be set to 1. If the logical address corresponds with a physicaladdress where the memory to be accessed is expected to be greater than asingle data chunk, only the first data chunk may be located in theshadow memory, and the fC flag may be set to 0 to indicate that the nextdata chunk is located in the NAND flash.

Referring back to FIG. 3, the fS, fC, and shadow address portions of themapping table 600 may be generated during block 308 when data isshadowed to the DRAM. In this manner, the controller 111 of FIG. 1 mayprovide the fS, fC, and/or shadow address values and store them in amapping table stored in the DRAM, NAND flash, or combinations thereof.While a mapping table including logical to physical address associationsand physical to shadow address associations is shown in FIG. 6, in otherexamples multiple tables may be used (e.g. one for logical to physicaladdresses and another for physical to shadow address associations). Inother examples, data structures other than tables may be used torepresent some or all of the information shown in FIG. 6.

FIG. 7 is a schematic illustration of NAND flash and DRAM address spacesarranged in accordance with an embodiment of the present invention. TheNAND flash address space 705 is shown, which may be spread over one ormore NAND flash devices, such as the devices 115 of FIG. 1. The addressspace includes data chunks 706-708. These data chunks are illustrated ascontaining data less than a size of a data chunk that may have beenconsolidated during a data re-organization process described herein. Forexample, the data chunk 706 may include two 64 kB segments of data, eachof which is expected to be responsive to a respective memory command.Similarly, the data chunk 707 may include two 32 kB segments of data andone 64 kB segment of data, each of which is again expected to beresponsive to a respective memory command. The data chunk 708 mayinclude 5 16 kB segments, 2 8 kB segments, and 1 32 kB segment, each ofwhich is expected to be responsive to a respective memory command. Thedata segments shown in data chunks 706-708 have been arranged to bestored in a smallest number of possible data chunks, where the datachunk size shown in FIG. 7 is 128 kB. Because the data in the datachunks 706-708 are associated with memory accesses for data less than asize of a data chunk, the data chunks 706-708 may be shadowed to one ormore DRAM devices, for example in block 308 of FIG. 3. Accordingly, inFIG. 7, the data chunks 706-708 have been shadowed into DRAM addressspace 710 as data chunks 716, 717, and 718.

As has been described herein, the controller 111 of FIG. 1 may performthe shadowing shown in FIG. 7 and described with reference to FIG. 3.The controller 111 may further generate the mapping table, for examplethe mapping table 600 of FIG. 6. Referring to FIG. 7 again, when thedata chunk 706 is shadowed to DRAM as data chunk 716, the following mayresult in the mapping table. The logical address of each of the 64kBdata segments making up the data chunk 706 may be associated with therespective addresses in the data chunk 706 by storing those NAND flashaddresses in the physical address column of the logical to physicaladdress 602 portion of the mapping table associated with the correctlogical address. Moreover, because the data accesses in the data chunks716-718 are expected to be less than or equal to the size of a datachunk, the data has been shadowed, so the corresponding addresses of thedata segments in the data chunk 716 in DRAM address space 710 may bestored in the shadow address portion of the physical to shadow addresstable 604. The data is shadowed data, so the fS flags for the data maybe set (e.g. by the memory controller 111 of FIG. 1) to indicate thedata is shadowed. Moreover, because the data in the data chunks 716-718are associated with data accesses for data equal to or less than a sizeof a data chunk, the next data chunk to be read will come from the DRAMdevices as well (e.g. because the first portion of each memory accessmay be shadowed to DRAM), accordingly, the fC flags for the data in datachunks 716-718 may be set (e.g. by the memory controller 111 of FIG. 1)to indicate the next data chunk may be read from the DRAM.

Referring again to FIG. 7, data chunks 726-728 may correspond to a dataaccess for an amount of data greater than a size of a data chunk (e.g.three data chunks 726-728). As has been described above, the data chunks726-728 may have been re-organized to be aligned with a data chunkboundary and/or moved to accommodate other data reorganization.Accordingly, the physical address of the data chunks 726-728 may bestored in the logical to physical address table 602 of FIG. 6corresponding to the relevant logical addresses. Because the data accessis for an amount of data larger than a data chunk, only a first datachunk may be shadowed to the DRAM in some examples. Accordingly, thedata chunk 726 may be shadowed to the DRAM at data chunk 736. The fSflag for the physical address corresponding to the data chunk 726 may beset (e.g. by the controller 111) to indicate that the data chunk 726 isshadowed. However, the fS flag for the physical address corresponding tothe data chunks 727 and 728 may be set (e.g. by the controller 111) toindicate that the data chunks 727 and 728 are not shadowed. The shadowaddress corresponding to the DRAM data chunk 736 may further be storedin the physical to shadow address table 604. Because the data chunk 736pertains to a data access of greater than a data chunk, the fC flag maybe set (e.g. by the controller 111) to indicate that the next dataaccess may come from the NAND flash devices.

In examples of the present invention, training, data re-organization,and shadowing, examples of which have been described with reference toFIGS. 3-7, may occur during power-up of a memory system, prior to memorysystem distribution (e.g. during manufacture), or at other scheduledtimes. While training, data re-organization, and/or shadowing may insome examples occur dynamically during operation, dynamic operation maynot be advantageous in applications desiring to minimize latencyassociated with accessing the stored data.

During operation of the exemplary memory system of FIG. 1, thecontroller 111 may perform shadowing during power-up as described withreference to FIG. 3. The controller 111 may store an indication in astatus register or provide a busy signal to the processor-based system105 indicative of the unavailability of some or all of the memory system107 functionality due to shadowing operations. When shadowing iscompleted, the controller 111 may store an indication in a statusregister or provide a signal to the processor-based system 105indicative of availability of the memory system 107.

On receipt of a memory command from the processor-based system 105, thecontroller 111 may identify a logical address associated with the memorycommand and access a mapping table (e.g. the logical to physical addresstable 602 of FIG. 6) to identify a corresponding physical address to thereceived logical address. The controller 111 may additionally or insteadaccess the fS flag corresponding to the logical/physical address pair.If the fS flag is indicative that the requested data is not shadowed,the controller may access the physical address associated with thereceived logical address to access the data. If the fS flag isindicative that the requested data is shadowed, the controller mayaccess the shadow address associated with the received logical address(e.g. as stored in the physical to shadow address table 604) and mayrequest the data from the shadowed address in DRAM. The controller mayfurther access the fC flag associated with the shadowed address. If thefC flag is indicative that the next data chunk is in the NAND flash, thecontroller may access the next data chunk from NAND flash memory duringa time the first data chunk is being accessed from the DRAM and/orprovided to the processor-based system 105. If the fC flag is indicativethat the next data chunk is in the shadow memory (e.g. DRAM), thecontroller may not begin accessing NAND flash and instead access a nextarea in DRAM and/or await a next memory command from the processor-basedsystem 105.

In this manner, memory systems may include NAND flash memory and DRAMmemory. Portions of the NAND flash memory may be shadowed to the DRAMmemory such that initial portions of memory accesses may be retrievedfrom the DRAM memory with generally a lower latency than they would havebeen retrieved from the NAND flash memory. The process of accessinglater portions of the memory access from NAND flash may begin while theinitial portion or portions are being retrieved from DRAM memory and/orprovided to a host. In this manner, latency associated with the NANDflash from the perspective of the host may be reduced.

Embodiments of the present invention may further provide for varioushousekeeping operations (e.g. wear leveling, bad block replacement) tobe performed only at particular times, e.g. power on of the system. Insome examples, referring back to FIG. 1, the controller 111 may conducterror correction operations on data written to and/or read from the NANDflash devices 115. In the event bad memory regions are identified duringthe error correction operations, the controller 111 may log those badmemory regions in a management log, which may be stored, for example inone or more of the NAND flash devices 115 and/or in one or more of theDRAM devices 125. The controller 111 may not take action to immediatelycorrect a bad memory region once identified. Instead, in some examples,when the memory system 107 is powered on, or at another specified time,the controller 111 may correct all bad memory regions stored in themanagement log which had been logged since the last correction tookplace. Similarly, the controller 111 may count reads and/or writes tothe NAND flash devices for the purposes of wear leveling. Instead ofacting immediately on identifying a memory region which should bedesignated as bad based on a wear leveling count, the controller 111 maylog the memory regions requiring replacement due to wear leveling in amanagement log, which may be the same or different log as the logreferred to above with respect to error correction. The controller 111may then designate as bad memory regions those regions identified in themanagement log due to their wear leveling counts at a specified time,e.g. power up.

Accordingly, in some examples, bad block replacement, read disturb, wearleveling, and/or other management tasks may be performed by the memorycontroller 111 of FIG. 1 responsive to a power cycle (e.g. power up) ofthe memory system 107. At other times, the controller 111 may notperform these tasks, which may avoid additional latency problemsassociated with these housekeeping operations.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

What is claimed is:
 1. A memory system comprising: a controllerconfigured to receive memory commands from a processor-based system; amemory device of a first type coupled to the controller over a firstinterface; a memory device of a second type coupled to the controllerover a second interface, wherein the memory device of a second type hasa lower latency than the memory device of the first type; and a datastructure stored in the memory device of the first type and/or in thememory device of the second type, wherein the data structure includesassociations between logical addresses received from the processor-basedsystem, physical addresses of the memory device of the first typeassociated with the logical addresses, shadow addresses of the memorydevice of the second type when data is stored in both memory devices ofthe first and second types, and a first flag indicating whether dataassociated with the physical address of the memory device of the firsttype is shadowed to the memory device of the second type, wherein thecontroller is configured to operate during a training period to generatethe data structure of starting addresses and lengths of data associatedwith memory commands from the processor-based system, and access aninitial portion of data corresponding to at least one of the startingaddresses stored on the memory device of the first type at least in partduring a time that a remaining portion of data associated the at leastone of the starting addresses is accessed on the memory device of thesecond type.
 2. The memory system of claim 1, wherein the controller isconfigured to receive a signal from the processor-based systemindicative of a start of the training period.
 3. The memory system ofclaim 1, wherein the controller is configured to shadow the initialportion of data from the memory device of the first type to the memorydevice of the second type.
 4. The memory system of claim 1, wherein thememory device of the first type comprises a NAND flash memory device. 5.The memory system of claim 1, wherein the memory device of the secondtype comprises a DRAM memory device.
 6. The memory system of claim 1,wherein the data structure is stored in a memory accessible to thecontroller.
 7. The memory system of claim 1, wherein the controller isfurther configured to generate the data structure based on an analysisof data stored in the memory device of the first type.
 8. The memorysystem of claim 1, wherein the controller is further configured togenerate the data structure based on receipt of read commands from theprocessor- based system during the training period.
 9. The memory systemof claim 1, wherein the processor-based system comprises a gamblingmachine.
 10. The memory system of claim 1, wherein the data structure isa list.
 11. A memory system comprising: a controller; a memory device ofa first type coupled to the controller over a first interface; a memorydevice of a second type coupled to the controller over a secondinterface, wherein the memory device of the second type has a lowerlatency than the memory device of the first type; and a data structurestored in the memory device of the first type and/or in the memorydevice of the second type, wherein the data structure includesassociations between logical addresses received from a processor-basedsystem, physical addresses of the memory device of the first typeassociated with the logical addresses, shadow addresses of the memorydevice of the second type when data is stored in both memory devices ofthe first and second types, and a first flag indicating whether dataassociated with the physical address of the memory device of the firsttype is shadowed to the memory device of the second type, wherein thecontroller is configured to receive memory commands from theprocessor-based system, wherein data corresponding to memory commandshaving a length less than or equal to a data chunk are located on boththe memory device of the first type and the memory device of the secondtype, wherein a first data chunk of data corresponding to memorycommands having a length greater than the data chunk are located on boththe memory device of the first type and the memory device of the secondtype, and wherein the controller is configured to provide the first datachunk corresponding to memory commands having the length greater thanthe data chunk from the memory device of the second type and access anext data chunk of data from the memory device of the first type atleast in part during a time the first data chunk is being accessed,provided to the processor-based system, or combinations thereof.
 12. Thememory system of claim 11, wherein the data structure further includes asecond flag indicating whether a next data chunk associated with theshadow address is stored in the memory device of the first type or thememory device of the second type.
 13. The memory system of claim 11,wherein the memory device of the first type comprises a NAND flashdevice and the memory device of the second type comprises a DRAM device.14. A method for servicing memory commands, the method comprising:receiving a memory command from a processor-based system associated withrequested data having a size greater than a data chunk; accessing afirst data chunk of the requested data from a memory device of a firsttype, wherein accessing the first chunk comprises reading a flag from adata structure indicative of the first chunk being shadowed to thememory device of the second type; providing the first data chunk to theprocessor-based system; accessing a second data chunk of the requesteddata from a memory device of a second type at least in part during atime the first data chunk is being accessed, provided to theprocessor-based system, or combinations thereof, wherein the memorydevice of the second type has a latency higher than a latency of thememory device of the first type; and providing the second data chunk tothe processor-based system.
 15. The method of claim 14, wherein the datachunk has a size such that by a time the first data chunk has beenprovided to the processor-based system, the second data chunk is readyfor provision to the processor-based system.
 16. The method of claim 14,wherein accessing the first chunk comprises accessing an associationbetween a logical address received in the memory command and a physicaladdress on the memory device of the first type.